Single crystal group III nitride semiconductors represented by the composition formula AlAGa1-AN (0≤A≤1) can freely select the emission peak wavelength in the range of 210 to 365 nm by changing the composition of the group III element (Al, Ga), and have direct transition-type band structures in the energy range corresponding to the above-described wavelength range. Thus, they are optimal materials for forming ultraviolet light-emitting elements.
Ultraviolet light-emitting elements formed of group III nitride semiconductors are generally manufactured by performing crystal-growth of an AlAGa1-AN layer on a single crystal substrate by a metal organic chemical vapor deposition method (MOCVD) or a molecular beam epitaxy method (MBE). As a single crystal substrate, materials such as sapphire, SiC (silicon carbide), AlN (aluminum nitride), or the like, which enable epitaxial growth of an AlAGa1-AN layer, may be used. Among these, it is known that use of an AlN single crystal substrate which is the same group III nitride as the AlAGa1-AN layer can reduce crystal defects (dislocations) formed in the AlAGa1-AN layer.
For example, Non-Patent Literatures 1 and 2 disclose that the dislocation density in the AlAGa1-AN layer can be reduced by fabricating an ultraviolet light-emitting diode including a stacked structure having an AlAGa1-AN layer lattice-matched to, and formed on, an AlN single crystal substrate. Non-Patent Literatures 1 and 2 disclose an ultraviolet light-emitting diode having an emission peak wavelength of 280 nm or less. When a flip-chip type ultraviolet light-emitting element described in these Non-Patent Literatures 1 and 2 is fabricated, it is preferable that the n-type AlAGa1-AN layer formed on the AlN single crystal substrate have high conductivity and a large film thickness in order to reduce the driving voltage of the ultraviolet light-emitting element.
Specifically, the film thickness of such an AlAGa1-AN-layer has been reported as follows. For example, Non-Patent Literature 3 reports the physical properties of an AlAGa1-AN layer formed on an AlN single crystal substrate. Specifically, when the Al0.6Ga0.4N layer (Al composition (A) is 0.6) has a film thickness of 0.5 μm or less, and when the Al0.7Ga0.3N layer (Al composition (A) is 0.7) has a film thickness of 1.0 μm or less, the lattice constant of the AlAGa1-AN layer is maintained to be equal to (lattice-matched to) the lattice constant of the AlN single crystal substrate. Non-Patent Literature 3 also reports that, if this condition is satisfied, AlAGa1-AN layers having good surface smoothness and high crystal qualities can be obtained.
In addition, Patent Literature 1 discloses an AlAGa1-AN layer which is formed on an AlN single crystal substrate, and whose film thickness is 5 times or more the predicted critical film thickness calculated in accordance with the Matthews-Blakeslee theory (hereinafter, referred to as MB theory in some cases) and whose mean dislocation density is 104 cm−2 or less (Note that Patent Literature 1 describes the unit of the predicted critical film thickness as Å in the drawing showing the relation between the Al composition and the predicted critical film thickness by the MB theory, but it is presumed that it is wrong, and should be nm (nanometer)).
As described above, Non-Patent Literature 3 and Patent Literature 1 disclose relatively thick n-type AlAGa1-AN layers formed on, and lattice-matched to, the AlN single crystal substrate.